AQA Unit 3: Chemistry 3

C3.1: The periodic tableC3.2: Water

C3.3: Energy calculationsC3.4: Analysis and synthesis

C3.5: The production of ammoniaC3.6: Alcohols, carboxylic acids and esters

C3.1: The periodic table

C3.1.1: The early periodic tableC3.1.2: The modern periodic table

C3.1.3: Trends within the periodic table

The early periodic table

• Mendeleev:– Also arranged elements according to atomic weights– Elements repeat properties periodically (regularly), also

placed in columns, or groups– Gaps left for undiscovered elements– Three new elements discovered in his lifetime that fitted

these gaps

C3.1.1

• Newlands:– Arranged elements according to atomic weights– ‘Law of Octaves’ similar properties repeated every 8th element– Some elements placed in inappropriate groups, due to strictly

following order of weights– Sometimes more than one element in each ‘position’

The modern periodic table

• Electrons, protons and neutrons were discovered early in the 20th century

• Elements now arranged by atomic (proton) numbers• All elements now in appropriate groups• Elements in the same group have the same number of

electrons in their highest occupied energy level (outer shell)

C3.1.2

Group 1

3Li 2, 1

11Na 2, 8, 1

19K 2, 8, 8, 1

37Rb 2, 8, 18, 8, 1

55Cs 2, 8, 18, 18, 8, 1

Group 2

4Be 2, 2

12Mg 2, 8, 2

20Ca 2, 8, 8, 2

38Sr 2, 8, 18, 8, 2

56Ba 2, 8, 18, 18, 8, 2

Trends within the periodic table:Group 1 - the alkali metals

• Physical properties:– Low density (the first three elements in the group are less dense than

water)– The further down the group an element is:

• the more reactive the element• the lower its melting point and boiling point

• Chemical properties:– All react with non-metals to form ionic compounds in which the

metal ion carries a charge of +1– The compounds are white solids that dissolve in water to form

colourless solutions– All react with water, releasing hydrogen– All form hydroxides that dissolve in water to give alkaline solutions

C3.1.3

Trends within the periodic table:The transition elements

• Compared with the elements in Group 1, transition elements:– have higher melting points (except for mercury) and

higher densities– are stronger and harder– are much less reactive and so do not react as

vigorously with water or oxygen• Many transition elements have ions with

different charges, form coloured compounds and are useful as catalysts

C3.1.3

Trends within the periodic table:Group 7- the halogens

• The elements in Group 7 of the periodic table react with metals to form ionic compounds in which the halide ion carries a charge of –1

• In Group 7, the further down the group an element is:– the less reactive the element– the higher its melting point and boiling point

• A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.

C3.1.3

Trends within the periodic table:Explaining the trends

• The trends in reactivity within groups in the periodic table can be explained because the higher the energy level of the outer electrons:– the more easily electrons are lost– the less easily electrons are gained

• Group 1 elements react by losing an electron– Therefore Li is less reactive than K, because it its outer

electron is nearer to the attractive nucleus• Group 7 elements react by gaining an electron– So F2 is more reactive than I2, because its outer electrons are

nearer the positive nucleus

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C3.2: Water

C3.2.1: Hard and soft waterC3.2.2: Purifying water

Hard and soft water

• Hard water contains dissolved compounds, usually of calcium or magnesium ions– The compounds are dissolved when water comes into

contact with rocks• There are two types of hard water– Permanent hard water remains hard when it is boiled – Temporary hard water is softened by boiling– Higher Tier: Temporary hard water contains

hydrogencarbonate ions (HCO3-) that decompose on

heating to produce carbonate ions which react with calcium and magnesium ions to form precipitates

C3.2.1

Hard and soft water:consequences

• Soft water readily forms lather with soap• Hard water reacts with soap to form scum and so more

soap is needed to form lather– Soapless detergents do not form scum

• Using hard water can increase costs because more soap is needed. When temporary hard water is heated it can produce scale that reduces the efficiency of heating systems and kettles

• Hard water has some benefits, because calcium compounds are good for the development and maintenance of bones and teeth and also help to reduce heart disease

C3.2.1

Hard and soft water:Removing hardness

• Hard water can be made soft by removing the dissolved calcium and magnesium ions. This can be done by:– adding sodium carbonate, which reacts with the

calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate

– using commercial water softeners such as ion exchange columns containing hydrogen ions or sodium ions, which replace the calcium and magnesium ions when hard water passes through the column

C3.2.1

Purifying water

• For humans, drinking water should have sufficiently low levels of dissolved salts and microbes

• Water filters containing carbon, silver and ion exchange resins can remove some dissolved substances from tap water to improve the taste and quality

• Chlorine may be added to drinking water to kill microbes and fluoride may be added to improve dental health (pros and cons?)

• Pure water can be produced by distillation

C3.2.2

NB: Detailed knowledge of specific water filters is notrequired.

C3.3: Energy calculations

C3.3.1: Energy from reactions

Energy from reactions:Comparing the energy released by fuels

• The relative amounts of energy released when substances burn can be measured by simple calorimetry, e.g. by heating water in a glass or metal container

C3.3.1

• Using: Q = mc ΔT, where:Q = energy, in Joules (J)m = mass of substance heated

(usually water)c = specific heat capacity (energy

required to heat one gram of a substance up by 1°C)

ΔT = change of temperature

Energy from reactions:Energy transfers in solution

• The amount of energy released or absorbed by a chemical reaction in solution can be calculated from the measured temperature change of the solution when the reagents are mixed in an insulated container

• This method can be used for reactions of solids with water or for neutralisation reactions

• Again, this will use Q = mc ΔT, but this time ‘m’ is the mass of both solutions (or solid + water) used

C3.3.1

Energy from reactions:Energy level diagrams

• Simple energy level diagrams show the relative energies of reactants and products, and can show activation energy and the overall energy change of a reaction (ΔH)

C3.3.1

energy

Exothermic

energy

reactants

products

-ΔH

time

Endothermic

energy

reactants

products

+ΔH

timeEndothermic,with activation energy (Eact)

reactants

products

+ΔH

Eact

time

Energy from reactions:Calculations using bond energies

• During a chemical reaction:– energy must be supplied to break bonds– energy is released when bonds are formed

ΔH = sum of bonds broken – sum of bonds made

• In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds: (-ΔH)

• In an endothermic reaction, the opposite is true: (+ΔH)

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Energy from reactions:Catalysts & fuel issues

• Catalysts provide a different pathway for a chemical reaction that has a lower activation energy

• Hydrogen can be burned as a fuel in combustion engines:

hydrogen + oxygen water– So, no greenhouse gases produced– But, energy is needed to produce the hydrogen

• It can also be used in fuel cells that produce electricity to power vehicles.

C3.3.1

NB: Knowledge of the details of the reactions in fuel cells is not required

C3.4: Analysis and synthesis

C3.4.1: Analysing substances

Analysing substances: Tests for positive ions – flame tests

• Many metal ions will produce distinctive colours when a crystal or a solution of a metal compound is held in a flame:– lithium compounds result in a crimson flame– sodium compounds result in a yellow flame– potassium compounds result in a lilac flame– calcium compounds result in a red flame– barium compounds result in a green flame

C3.4.1

Analysing substances: Tests for positive ions - NaOH

• Some metal ions will form a precipitate (solid) when added to solution of sodium hydroxide (NaOH(aq))– Aluminium, calcium and magnesium ions form white

precipitates with sodium hydroxide solution• only the aluminium hydroxide precipitate dissolves in excess

sodium hydroxide solution

– Copper(II) (Cu2+) forms a blue precipitate– Iron(II) (Fe2+) forms a green precipitate (which quickly

turns brown due to oxidation with oxygen in the air)– Iron(III) (Fe3+) forms a brown precipitate

C3.4.1

Analysing substances: Tests for negative ions

• Carbonates react with dilute acids to form carbon dioxide:– Carbon dioxide produces a white precipitate with limewater,

which turns the limewater cloudy• Halide ions in solution produce precipitates with silver

nitrate solution in the presence of dilute nitric acid:– Silver chloride is white– Silver bromide is cream– Silver iodide is yellow

• Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid

C3.4.1

Analysing substances: Titrations

• The volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator

• You should be able to describe a titration that would allow a successful result to be obtained, including:– solution of unknown concentration in (conical) flask, volume

measured using pipette– indicator in (conical) flask– Solution of known concentration in burette– white tile under flask (to make colours easier to see)– slow addition, drop-wise– Swirling the mixture (to ensure it mixes quickly)– colour change when end (neutralisation) point is reached– volume of known solution added is recorded

C3.4.1

Analysing substances: Titrations

• As the concentration of one of the reactants is known, the results of a titration can be used to find the concentration of the other reactant– You may also be asked to then convert it from mol/dm3 to g/dm3

• Method:1. Calculate number of moles of acid used2. Write balanced equation for reaction3. Using moles of acid information, deduce number of moles

of alkali required4. Calculate concentration of alkali• This process also works the same in reverse.

number of moles = volume (cm3) x concentration (mol/dm3)1000

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C3.5: The production of ammonia

C3.5.1: Making ammonia

Making ammonia

• The raw materials for the Haber process are nitrogen and hydrogen. – Nitrogen is obtained from the air– Hydrogen may be obtained from natural gas or other sources (e.g.

electrolysis of water)• The purified gases are passed over a catalyst of iron at a high

temperature (about 450 °C) and a high pressure (about 200 atmospheres). Some of the hydrogen and nitrogen reacts to form ammonia. The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen:

nitrogen + hydrogen ammonia• On cooling, the ammonia liquefies and is removed• The remaining hydrogen and nitrogen are recycled

C3.5.1

Equilibrium – what is it?

• When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction

• The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction, such as:– Temperature– Pressure

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Equilibrium – effect of temperature

• If the temperature is raised, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases

• If the temperature is lowered, the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases

• The Haber process is exothermic, so a higher temperature produces less ammonia!

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Equilibrium – effect of pressure

• In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction.

• In the case of the Haber process:N2 + 3H2 2NH3

– The right hand side is favoured by higher pressure, as it has two molecules, compared to four

– So higher pressure means higher yield of ammonia

C3.5.1

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C3.6: Alcohols, carboxylic acids and esters

C3.6.1: AlcoholsC3.6.2: Carboxylic acids

C3.6.3: Esters

Alcohols

• Alcohols contain the functional group –OH. Methanol (CH3OH), ethanol (C2H5OH) and propanol (C3H7OH) are the first three members of a homologous series of alcohols

• They all:– dissolve in water to form a neutral solution– react with sodium to produce hydrogen– burn in air (must be able to write balanced symbol equations for

combustion reactions)– are used as fuels and solvents, and ethanol is the main alcohol in

alcoholic drinks• Ethanol can be oxidised to ethanoic acid, either by chemical

oxidising agents or by microbial action. Ethanoic acid is the main acid in vinegar

C3.6.1

Structural formula:C2H5OH

Displayedformula:

H H

H H

C CH O H

Carboxylic acids

• Carboxylic acids have the functional group –COOH• Methanoic acid (CHOOH), ethanoic acid(CH3COOH) and

propanoic acid (C2H5COOH) are the first three members of the homologous series

• Carboxylic acids:– dissolve in water to produce acidic solutions– react with carbonates to produce carbon dioxide– react with alcohols in the presence of an acid catalyst to produce esters– do not ionise completely when dissolved in water and so are

weak acids– aqueous solutions of weak acids have a higher pH value than

aqueous solutions of strong acids with the same concentration

C3.6.2

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Structural formula:CH3COOH

Displayedformula:

H

H

C CHO

H

O

=

NB: You do not need to know any equations for these reactions

Esters

• Ethyl ethanoate is the ester produced from ethanol and ethanoic acid.

• Esters have the functional group –COO–• They are volatile compounds with distinctive

smells and are used as flavourings and perfumes

• You should be able to name ethyl ethanoate, and recognise if a compound is an ester from its name or its structural formula.

C3.6.3

Structural formula:CH3COOCH2CH3

Displayedformula:

H

H

C CHO

C

O

= H

H

C H

H

H